Examiners Report June GCE Chemistry 9CH0 03

Transcription

1 Examiners Report June 2017 GCE Chemistry 9CH0 03

2 Edexcel and BTEC Qualifications Edexcel and BTEC qualifications come from Pearson, the UK s largest awarding body. We provide a wide range of qualifications including academic, vocational, occupational and specific programmes for employers. For further information visit our qualifications websites at or Alternatively, you can get in touch with us using the details on our contact us page at Giving you insight to inform next steps ResultsPlus is Pearson s free online service giving instant and detailed analysis of your students exam results. See students scores for every exam question. Understand how your students performance compares with class and national averages. Identify potential topics, skills and types of question where students may need to develop their learning further. For more information on ResultsPlus, or to log in, visit Your exams officer will be able to set up your ResultsPlus account in minutes via Edexcel Online. Pearson: helping people progress, everywhere Pearson aspires to be the world s leading learning company. Our aim is to help everyone progress in their lives through education. We believe in every kind of learning, for all kinds of people, wherever they are in the world. We ve been involved in education for over 150 years, and by working across 70 countries, in 100 languages, we have built an international reputation for our commitment to high standards and raising achievement through innovation in education. Find out more about how we can help you and your students at: June 2017 Publications Code 9CH0_03_1706_ER All the material in this publication is copyright Pearson Education Ltd GCE Chemistry 9CH0 03

3 Introduction This is the first opportunity for candidates to sit paper 3 from the Pearson Edexcel Level 3 Advanced GCE in Chemistry. The paper tests understanding from across all parts of the specification and many of the questions are synoptic in nature. In addition, a central core of the paper covers the indirect assessment of practical skills. Questions in this context assess conceptual and theoretical understanding of experimental methods that will draw on candidates experiences of the core practicals. The main differences between this paper and the style of assessment used in the previous specification are: a much longer exam paper; a wider breadth of content; the indirect assessment of practical skills; greater emphasis on extended writing questions; greater emphasis on unstructured calculations; no multiple-choice questions; questions that will target mathematical skills at Level 2 or above. The paper counts as 40% of the total qualification and covers all three assessment objectives, AO1, AO2 and AO3 as outlined in the specification. It provides an opportunity for candidates to demonstrate knowledge of chemical principles and apply them to a wide range of both familiar and unfamiliar contexts, both quantitatively and qualitatively. It also will challenge candidates to show that they can analyse, interpret and evaluate information, often data or observations from a practical context. During this session, many of the scripts they saw at the higher levels impressed the examination team. It seemed that stronger candidates coped with the increased demand, in terms of the content covered by a single paper, and the changes in question style outlined. Particular strengths included: the ability to process data from an enthalpy experiment in question 4(b); use of skeletal formulae; use of data from mass spectra; some organic mechanisms e.g. the nucleophilic substitution mechanism in question 8 (a); the ability to discriminate between transition metals and metals such as zinc and scandium in question 7(a). GCE Chemistry 9CH0 03 3

4 However, the performance of candidates was less effective in other areas of the paper. These included: the knowledge of the reactions of copper and chromium in question 1; the ability to apply knowledge of a known mechanism to an unfamiliar context e.g. in question 8(b); the ability of some candidates to make significant progress in unstructured calculations; justifying how the results of experimental work may change under different circumstances. As a result, it is important for candidates to experience a wide range of practical work in preparation for this paper, and for them to ensure they reflect on both the procedures followed and the content covered in such work. 4 GCE Chemistry 9CH0 03

5 Question 1 (a) (i) In general, the chemistry of the transition metals was not well known, and the answers to this question illustrated that. Many ignored or did not appreciate the phrase amphoteric or the fact the hydrochloric acid was dilute and assumed this reaction was a ligand substitution involving chloride ions. Even those who realised it was a reaction with H + ions found it difficult to balance the equation correctly. In one sense, this type of question is not particularly demanding but it does show the importance of making sure the breadth of the course is appreciated as well as the depth. Although the correct chromium complex is shown, 3 additional waters are evident on the left hand side, and no state symbols are shown, despite the guidance in the question. Hence this example was not worth any credit. Make sure you learn the reactions of chromium as outlined in the specification, as they can be difficult to work out when required, without recall to fall back on. Question 1 (a) (ii) Most students appreciated the colours of the species involved in the reaction but lost marks by not using terms such as solution and precipitate. This is a vital distinction to make when describing such reactions. This example did not gain any marks. The candidate has not described physical states seen in the reaction and the colours given are incorrect. Remember to include descriptions such as solution and precipitate when appropriate, and not just describe change in colour. GCE Chemistry 9CH0 03 5

6 Question 1 (a) (iii) Candidates seemed to find this equation a bit more straightforward than the reaction in Q1(a)(i), though missing state symbols were again common. Other common errors included missing charges on the complex ion, and use of NaOH, rather than OH - in the question. Those who started with [Cr(OH) 3 (H 2 O) 3 ](s) often failed to balance the equation correctly. This example made one slip, the omission of the 3 before the hydroxide ion. Hence it gained 1 mark for the correct state symbols. Double check equations to make sure they balance. Question 1 (a) (iv) This question was often correct, though a minority simply stated green and so missed the mark. Remember, you can distinguish between a precipitate and a solution by the fact that although both may be coloured, a solution will still let light pass through. The colour is correct, but not the state, so no marks were given for this answer. 6 GCE Chemistry 9CH0 03

7 Question 1 (b) (i) The descriptions seen in this question were far better than those in (a), and candidates regularly used the terms solution and precipitate correctly. In addition, most answers distinguished between the shades of blue observed, and so could score both marks. In this case, the candidate has not recognised that the precipitate dissolves to form a dark blue solution, so only scores 1 of the 2 marks. Use colour coding in your revision notes for the transition metal topic, when writing out the formulae of complexes. This will help to act as a memory trigger. Question 1 (b) (ii) Despite the success of many candidates in part (b)(i), which implied that most had carried out this reaction, far fewer could recall the species involved and so write a balanced equation. In weaker responses they simply tried to write an equation for the reaction of copper sulfate with ammonia, while others tried to replace the water molecules with six ammonia ligands. The right hand side of the equation is incorrect as it shows an additional 2 hydroxide ions. It scores 1 mark for the correct reactants. When carrying out qualitative practical chemistry, make sure you write an equation in your notes for each reaction you can describe. GCE Chemistry 9CH0 03 7

8 Question 2 (a) Most candidates seemed familiar with distillation apparatus and were quick to notice the incorrect direction of water flow, often justifying the need for change in terms of the efficiency of cooling. The replacement of the funnel proved more difficult, with a minority suggesting the whole apparatus should be sealed to prevent escape of volatile compounds. Those that did recognise the need to close the set-up on the left hand side did not always specify what they would use (e.g. bung, stopper etc.), and so missed the mark. Thermometers were regularly mentioned but nearly always in the context of determining the boiling temperature, which was not relevant in this case. In this case, the need to change the water direction is justified, so scores marks 1 and 2. The thermometer here is used to measure the temperature, rather than seal the apparatus so no additional credit is given. Remember that when using distillation to remove products of oxidation as they are formed, it is more important to recognise the need to seal the apparatus at the side with the reaction flask to prevent escape of volatile compounds, than it is to record the boiling temperature. 8 GCE Chemistry 9CH0 03

9 Question 2 (b) Understanding and use of skeletal formulae seemed embedded and this question was well answered consequently. The most common error was the use of 'CH3' in the structures. In this example the candidate has correctly drawn both structures but has ignored or misunderstood the command in bold to use skeletal formulae, so only scores 1 mark. Look at the stem of the question for words or phrases in bold. They are there to help you focus your response. GCE Chemistry 9CH0 03 9

10 Question 2 (c) (i) The majority of answers were sufficient to score the mark here with the simple recognition that secondary alcohols cannot be oxidised to carboxylic acids. A few simply stated because it is a secondary alcohol which was deemed insufficient to score. Here the candidate seems to be confusing the practical procedures one might use in the partial and complete oxidation of primary alcohols and as a result believes heating under reflux will form the carboxylic acid, so did not score. Make sure that as well as remembering organic reactions, you can justify why they occur under a given set of conditions. Question 2 (c) (ii) Despite the mention of sulfuric acid in the stem, and the previous question making it clear that carboxylic acids do not form in the reaction, a surprising minority did not score the mark. Common wrong answers included a variety of organic acids or alcohols and H 2 Cr 2 O 7. The formula for sulfuric (IV) acid is given, rather than sulfuric (VI) acid, so does not score. Make sure you revise the names and formulae of the common inorganic acids studied previously at GCSE as they will be expected knowledge at A level. 10 GCE Chemistry 9CH0 03

11 Question 2 (c) (iii) The writing of balanced equations continues to differentiate well between candidates, even for simple inorganic reactions such as this question. Candidates did not appreciate state symbols well, perhaps not considering the solubility of the sodium sulphate product nor the information in the question about the sodium carbonate solid added. The examination team were disappointed to regularly see NaCO 3 as the formula for sodium carbonate The incorrect formula was such a fundamental error at this level that such responses did not score. Learn the formula of common ions used in the course and practice using them, alongside solubility rules, throughout the two years of study. GCE Chemistry 9CH

12 Question 2 (d) (i) Candidates almost all appreciated the need to comment on the peak at 1720 cm -1, but fewer made the link between the peak and the carbonyl bond in the ketone. Some candidates simply tried to match peaks from the Data Booklet regardless of the structure of the ketone. Hence some answers claimed that peaks were due to alkene or alkyne groups. In this case the relevant peak is identified but not linked to the bond that caused it, so only scores one of the two available marks. When discussing IR spectra try to discuss specific bonds that cause the transmittance, rather than simply naming functional groups. 12 GCE Chemistry 9CH0 03

13 Question 2 (d) (ii) Most candidates recognised that the broad peak at around 3400 cm -1 would disappear. However, although many realised this peak was due to the alcohol, only in the better responses could they justify their answer by referring to the removal of the alcohol in the distillation. Others misunderstood the separation role of the second distillation and thought that this was done to ensure any remaining alcohol was completely oxidised. As in part (d)(i), the relevant peak is again identified. However, its disappearance is not justified so this only scores one of the two available marks. Look out for command words and make sure you know what they mean. In this case the prediction had to be justified, which means you have to give evidence/reasons to support your prediction. GCE Chemistry 9CH

14 Question 2 (e) (i) This was well answered in the main. Those who failed to score seemed to have misinterpreted the question or ignored the how you would find wording. Such candidates simply gave the value of 86 and did not score unless they included further clarification This response primarily failed to score because it did not say how to find the molar mass, but stated (incorrectly) what it was. Read each question with care, to check you are attempting to answer what the question asks. 14 GCE Chemistry 9CH0 03

15 Question 2 (e) (ii) Most candidates ensured their fragments were positively charged, so many scored both marks. However, some did carry out rearrangements to the ions or ignored the instruction in bold to use displayed formulae, so lost some credit. This example shows both ions correctly but as structural formulae. As the question specifically wanted to test the skill of writing a displayed formula, as well as the fragmentation, this answer only scored 1 of the 2 marks. When writing organic formulae, always check whether or not a question specifies the type of formula required in the answer. GCE Chemistry 9CH

16 Question 2 (f) (i) Given its relatively straightforward nature, this question differentiated between candidates extremely well. Candidates had a lot to consider from counting carbon atoms to recognition that the other product was water. However, the most frequent error was in failing to use molecular formulae correctly, with candidates separating OH in the alcohol and/or writing structural formulae. This candidate has used the correct formulae for the alcohol and oxygen on the left hand side. The right hand side has an incorrect product (hydrogen). Hence it scores 1 mark out of 2. Practise writing equations for organic oxidation reactions. Use [O] rather than trying to include inorganic oxidising agents, but remember that such equations must balance. 16 GCE Chemistry 9CH0 03

17 Question 2 (f) (ii) This twist on a percentage yield calculation proved relatively challenging for many. Many responses simply stopped after scaling up the mass of the product to 3.44 g, taking into account the 62.5% yield. Other common errors included using 65% as the yield, getting the molar masses of the alcohol and ketone the wrong way round, or working out the molar masses incorrectly. The candidate has correctly determined the moles of ketone formed, appreciated the 1:1 ratio, and scaled up correctly to find the moles of alcohol needed. Unfortunately, the molar mass of alcohol is incorrect, so they did not score the second mark. Lay out your working in calculations clearly. That way, if you do make a mistake it s easier for examiners to give you credit for what you have done, as well as transferred errors for work you do subsequently. GCE Chemistry 9CH

18 Question 3 (a) Candidates found this difficult, even though it is a relatively low demand skill. The significance of amino acid was lost on many. Even those who appreciated that diaminohexanoic acid was the main part of the name often did not count the carbon in the COOH, so used 1,5-' as their prefix. The example does not score as one of the amino groups is numbered incorrectly. Practice converting the common names of amino acids into IUPAC names. 18 GCE Chemistry 9CH0 03

19 Question 3 (b) This question differentiated reasonably effectively. In weaker responses, candidates could often draw the correct structure in alkaline conditions, but could not draw the ester. However, the protonation of both amine groups was often absent in the second structure, even for stronger candidates. This did not seem to be because they did not appreciate the possible reaction, as often one of the amine groups was protonated. This answers scores 2 of the 3 marks, as for the third structure the candidate has shown a zwitterion rather than the ester formed by reaction of the acid group with methanol. Remember when you study organic reactions you are learning how a functional group reacts, not a specific single compound. For example in this question, the third structure is testing your generic understanding of the reactions of the -COOH and OH functional groups rather than specifically the chemistry of these amino acids. You need to spot the functional groups you study in unfamiliar compounds and apply the chemistry you have learnt. GCE Chemistry 9CH

20 Question 3 (c) Many candidates could demonstrate the understanding of the peptide link and so answered this question successfully. Those who failed to score generally did so because they either introduced an additional oxygen atom into the link, or drew a section of a polypeptide/ protein consisting of the two units. This example fails to score as the peptide link is incorrect. Be careful not to confuse an ester link with a peptide link. 20 GCE Chemistry 9CH0 03

21 Question 3 (d) In better responses, candidates took notice of the emboldened phrase acidic conditions, and were able to justify the greater attraction of lysine to the stationary phase in terms of its greater positive charge. The majority of other responses did score the attraction mark, but tried to justify it with generally vague statements related to intermolecular forces or polarity. Disappointingly, a number of candidates seemed not to have developed an understanding of chromatography beyond the idea that heavier molecules will take longer to move through the column. 'Interact with the polar stationary phase to a greater extent' was sufficient for the 2nd mark - but the justification in terms of lysine being 'more polar' was not enough for the 1st mark. Remember that under acidic or alkaline conditions amino acids protonate or deprotonate respectively to form ions. This will influence their properties under these conditions, such as the way they interact with a stationary phase in chromatography. GCE Chemistry 9CH

22 Question 4 (a) Many candidates did not use the data already shown in the table as a model for the number of decimal places they should use. Hence a mark was often lost for the use of '1.6' for the mass of methanol. This candidate tried to use the data as a model, but focussed on significant figures, rather than decimal places as guided in the question. They did however score 1 mark for the correct precision for the temperature change data. Use the guidance in the question to help decide when decimal places or significant figures are more important. 22 GCE Chemistry 9CH0 03

23 Question 4 (b) More able candidates found this straightforward and often scored 2 marks. However some errors were common and these included: incorrect balancing of oxygen using 2O 2 ; use of (aq) for the state symbol of methanol; no written equation for 1 mole of methanol. This example scored 1 mark for the correct state symbols for the correct species but in an unbalanced equation. Take care when balancing combustion equations with alcohols, as you need to take into account the single oxygen atom in the hydroxyl group, when you deduce how many moles of elemental oxygen molecules are required. GCE Chemistry 9CH

24 Question 4 (c) This proved to be the most accessible unstructured calculation on the paper, with many candidates scoring 3 or 4 marks. Common errors included: use of 1.6 g when calculating energy; missing signs on the value for the enthalpy change; final answer given to 5 significant figures. This candidate scored the first 3 of the 4 marks available. They lost credit because they gave their final answer to 5 significant figures, which is not appropriate given that the least precise data in the question was to 3 significant figures. Use the least precise data in the question as a guide to the level of precision needed for your final answer. 24 GCE Chemistry 9CH0 03

25 Question 4 (d) (i) Most candidates could calculate the percentage uncertainty correctly. The most common way to not score was to round the final answer incorrectly, for instance giving an answer of 0.66% rather than 0.67%. This candidate has the correct working but has rounded their final answer incorrectly, so did not score the 1 mark available. Remember to check your rounding on the final answer in any calculation. GCE Chemistry 9CH

26 Question 4 (d) (ii-iii) In part (d)(ii), most candidates could identify that the percentage uncertainty when using the smaller measuring cylinder six times was greater, and so less preferable to use. This was often accompanied by the correct value for the percentage uncertainty, scoring 2 of the 3 marks. Comments on the practicalities of using the two types of cylinder were far less frequent, so the award of 3 marks was relatively rare. It was evident that candidates were familiar with this type of experiment and as a result, many could discuss at least two of the possible errors that may lead to an inaccurate value in part (d)(iii). Heat loss to the surroundings was by far the most common correct suggestion. Deviating from standard conditions was often cited as a reason, but was not given credit. Errors of the procedure were required, rather than external conditions under which the experiment was carried out. 26 GCE Chemistry 9CH0 03

27 In part (d)(ii), the percentage uncertainty is correctly calculated and identified as being greater than that of the larger cylinder used once, so this response scores 2 marks. The loss of heat to the surroundings as well as the evaporation of the fuel are evident, scoring 2 marks in part (d)(iii). When asked to give more than one reason for an answer, check to see that you are not giving the same answer more than once. For instance heat loss to the air and heat loss to the calorimeter are essentially the same point so worth only 1 mark. GCE Chemistry 9CH

28 Question 4 (e) Candidates seemed to find it hard to visualise any difference in the procedure followed when student 1 repeated the experiment. As a result, many answers suggested the value for the enthalpy of combustion would be the same. Those who did realise the temperature change would be lower could not always explain why, nor relate this with sufficient care to the effect on the enthalpy change. For instance, answers that just referred to the enthalpy change being lower did not score without further clarification. This candidate was careful to describe the change in the enthalpy of combustion as less exothermic rather than just lower so scored 1 mark. Unfortunately they have not explained why the temperature difference was not as big, so just missed out on another mark. When describing changes in enthalpy, try to use phrases like less exothermic and more exothermic rather than just greater or smaller. 28 GCE Chemistry 9CH0 03

29 Question 4 (f) As in (f) it seemed that candidates could not always appreciate the similarities and differences in the two procedures. As a result, many candidates simply homed in on the increase in heating time, so assumed incorrectly that the enthalpy change calculated would be more exothermic. Those who did appreciate that the energy was proportional to the amount of fuel burned nearly always scored both marks. This answer scored 1 mark for the recognition that the enthalpy change would be less negative. However, the reasoning was not enough to score the second mark. In this type of explain question, try to make a judgement on the value and then give reasons for your decision. GCE Chemistry 9CH

30 Question 4 (g) This question was by far the best answered in this section, which seems a little surprising as it does not directly relate to the procedure. However, this did not worry candidates who nearly all recognised the amount of calculated moles of methanol would be less, so the enthalpy change would be more exothermic. While terms such as greater were seen (and given credit), the preferable use of more exothermic in part (g) was seen far more than the required term less exothermic in part (e). This was an unusual example in that the candidate does not attempt to justify their choice. In this question the state mark was dependent on the justification, so this response was given 0 marks. When asked to justify an answer remember to give reasons or evidence for your choice. 30 GCE Chemistry 9CH0 03

31 Question 5 (a) (b) Some candidates perhaps overlooked part (a) since a number were blank. Those who did could not see past C δ+ used frequently in other parts of the specification. Many who had the required negative charge did not complete the dipole on magnesium to score the mark. Writing the systematic name in part (b) was more discriminating than expected, with the most common being numbering of the methyl group, which was often given the positional prefix of 4. Other candidates drew the correct alcohol. However, they then named it using whatever portion of the chain was shown horizontally on their diagram, which did not always consist of 5 carbons. This was a shame at it seemed that they had done the more difficult task, then lost a mark for essentially an AS-level skill. GCE Chemistry 9CH

32 This example loses the naming mark in part (b) as they have numbered the positions of the methyl and alcohol group incorrectly. Even though you probably study the rules for naming organic compounds quite early in your course, make sure you practise them at every opportunity throughout your studies. 32 GCE Chemistry 9CH0 03

33 Question 5 (c) (d) Quite a few candidates seemed to guess an answer to part (c). Some candidates did show great ingenuity in identifying propane in part (d), but the majority assumed it would be an alcohol or gave a structure containing MgOHBr. This example uses the idea that MgOHBr is often produced when Grignard reagents react to deduce the product correctly. Make sure you can deduce products from a range of reactions involving Grignard reagents. Use the idea of the inorganic product, MgOHBr, to help deduce the organic product. GCE Chemistry 9CH

34 Question 6 (a-d) Many candidates were aware of the level of precision required for titres in (a), so scored both marks. Candidates did sometimes slip and quote a value of '23.2' for titration 3. In part (b) the best scripts did score the mark. Others tended to get the change of colour the wrong way round or described a change of pink to pale pink. In practical terms, to the naked eye, solutions of Mn 2+ ions of this concentration are colourless. The calculation in part (c) also discriminated effectively. Most could pick up one or two marks for the amounts of MnO 4- of NO 2- respectively and some successfully managed to scale up the amount of NO 2- ions; a common error here was to assume the scaling factor was 250/25 rather than 250/average titre. At this point only the more able candidates made further progress using a variety of creditworthy strategies. The most common was to calculate the molar mass of the hydrated salt. However, some failed to recognise that the amount of the hydrated salt was half the amount of the NO 2- ions. Others used the more complex route of using the amount of hydrated salt to calculate the mass of all species present but water, then subtract these masses from the original mass of the hydrated salt. This enabled them to find the amount of water, which could then be compared to the amount of the salt to find x. Most candidates managed to score at least 1 mark in part (d), the common mistakes being omission of charges on ions and not simplifying the equation by cancelling out species such as water molecules. 34 GCE Chemistry 9CH0 03

35 GCE Chemistry 9CH

36 Here the data is quoted to an appropriate level of precision and the average titre calculated correctly, so part (a) scores 2 marks. The colour change in part (b) is the wrong way round, so does not score. In part (c) the candidate does not recognise that the amount of salt is half that of the NO 2- ions. This gives a molar mass of approximately 85, which, if carried forward would give a negative value for x, so is worth no further credit. In part (d) the 2:5 ratio is evident, so scores 1. The number of water molecules is incorrect and a charge is missing from one of the ions, so the 2nd mark could not be given. Label all your working clearly in unstructured calculations. Then if you do make a mistake, the examiner can follow what you are trying to do. 36 GCE Chemistry 9CH0 03

37 Question 6 (e) (f) In part (e) most candidates suggested that the amount or mass of magnesium carbonate would be less than expected. However, this very often led to a judgement that x would also be lower. In part (f) there seemed to be some confusion between the terms evaporate and decompose, with some suggesting that magnesium carbonate might do both, so did not score. A minority of candidates did not score because they referred to generic impurities in the residue rather than specific examples evident from the question, such as sodium carbonate. Only a small number of candidates made a judgement about the quality of the plan. GCE Chemistry 9CH

38 This candidate realises that the mass of magnesium carbonate would be less and correctly links this to the change in x, so scores 2 marks in part (e). In part (f) the impurities was not specific enough to gain credit. When evaluating a plan or procedure try to make comments that apply specifically to that experiment. Question 6 (g) (h) Some candidates appeared to confuse Method 3 with the reaction between magnesium and oxygen to find the formula of magnesium oxide. As a result, comments about lifting the crucible lid periodically were quite common in part (g). Others realised that you had to determine when all the water had been removed, but suggested inappropriate techniques such as testing for water with cobalt chloride paper. Near misses tended to include the idea of prolonged heating or of checking the mass, but not both. Part (h) was often well answered, though just the phrase use a more accurate balance was not enough to score a mark. 38 GCE Chemistry 9CH0 03

39 Although in part (g) the response uses the idea of more heating, it is not linked to the measurement of the mass over time, so does not score. In part (h), using a larger mass of the salt scores 1 mark. Remember that comments about repeating results are linked to the reliability of the data, but will not influence the percentage uncertainty. GCE Chemistry 9CH

40 Question 7 (a) The quality of extended responses in this question was generally good. At the higher level of achievement, answers scoring 5 or 6 marks were common, any error tending to be the use of shortened electron configurations such as [Ar]3d 10. On occasion, imprecise terminology such as Zn 2+ has a full d orbital as a way of justifying why zinc is not classified as a transition metal hindered a candidate s progress. Fortunately many went onto clarify this later in their answer with clear references to either full subshells, d orbitals are full or clear reference to d 10 in their answer. In a similar way, some answers used the imprecise term d-shell throughout, and so lost 1 mark. Many good answers also went on to justify the lack of colour in Zn 2+ and Sc 3+ solutions, which was not required by the question. Fortunately, most of this additional information was correct; the only problem it may have created was to leave candidates with less time available for later questions. In the weakest answers, candidates did at least realise that zinc and scandium are not transition metals but then tried to define transition metals as atoms with an incomplete d subshell, making it difficult to then access the points needed to explain their choice of scandium and zinc. 40 GCE Chemistry 9CH0 03

41 This example showed evidence of 3 indicative points, scoring 2 marks for these points. Answers that make 3 or 4 relevant indicative points, in general, score 1 reasoning mark. This makes 3 marks in total. The points made are: successfully defines transition metal; recognises Sc and Zn are not transition metals; recognises the full d-subshell in the Zn 2+ ions. Unfortunately the electron configurations of the ions are not evident and there is no reference to the empty d-subshell in Sc 3+, only that it is not incompletely filled, which could of course mean full. Make sure you are clear about the differences between orbitals, subshells and shells. GCE Chemistry 9CH

42 Question 7 (b) The lack of structure in calculations at this level provided both a degree of challenge and differentiation by outcome. In weaker responses, candidates benefitted by adopting a logical approach and in general looked to use the data to find the amounts of Cr 2 O 7 2- ions and Mn 2+ ions, so scored the first two marks. Many also went on the find the whole number ratio between the reactants. The better responses made further progress, using a variety of techniques to arrive at the correct outcome. These included use of oxidation numbers, deducing the numbers of electrons transferred by 3 moles of Mn 2+ and calculations of the number of moles of electrons. Occasionally in the final step, candidates added the 2 electrons to Mn 2+, getting an oxidation state of 0, so missing the final mark. 42 GCE Chemistry 9CH0 03

43 In this example, worth full marks, the candidate uses the half equation and the amount of Cr 2 O 7 2- ions to deduce the moles of electrons gained. They then compare this to the moles of Mn 2+ to deduce the electrons lost by each Mn 2+ ion, and so find the correct oxidation state at the end of the reaction. In an unstructured calculation based on quantitative chemistry, always start by calculating the amounts of substances present, using data from the question. GCE Chemistry 9CH

44 Question 7 (c) As in Q7(b), in the weaker responses candidates found the unstructured nature of the problem very challenging and rarely scored more than one mark. Often they were distracted by the idea of the cell and spent valuable time drawing a labelled diagram of the cell. The data in the question was often ignored. Instead a significant minority of candidates looked up and attempted to use E cell data. Sometimes this enabled candidates to narrow down the list of possible metals, and then by trial and error, identify X as magnesium, by working backwards from the atomic mass. Such an approach could score credit but was not efficient in terms of time used. Others successfully determined the amount of chromium, but did not realise the significance of the formula XSO 4, so could not deduce the mole ratio. Those who did deduce the ratio invariably went on to score full marks. 44 GCE Chemistry 9CH0 03

45 In this case the candidate has realised that X must form an X 2+ ion, narrowing down their choice to 3 possibilities, possibly based on their ability to reduce Cr 3+. However rather than trying to work backwards from the A r values of each metal, as some did, they use the more efficient method of deducing the amount of X and then calculating A r. Even when you have a data booklet, if you are given data in the question, you are expected to use it in your answer. GCE Chemistry 9CH

46 Question 8 (a) This question is on the nucleophilic substitution mechanism. Candidates were precise about the positioning of arrows. The use of correct charges on the intermediate and the final arrow from the N-H bond suggested that such candidates understood the principle of the curly arrow. In weaker responses the final arrow often went incorrectly to the hydrogen, which perhaps indicates candidates having a rote learning approach to mechanisms. The majority of candidates used the SN2 mechanism. This example scored 2 marks out of 3. They have correctly shown the lone pair on ammonia and the first two curly arrows are correct. The structure of the intermediate gets the second mark, but although the candidate realises that the intermediate must lose a hydrogen, perhaps they do not appreciate that the curly arrow represents a pair of electrons, as they try to donate two pairs of electrons to the hydrogen. As well as learning the mechanisms, try to write out descriptions in words when you revise. This will help you understand what the arrows represent, so might help you avoid getting curly arrows the wrong way round. 46 GCE Chemistry 9CH0 03

47 Question 8 (b) Candidates struggled to apply their knowledge of nucleophilic addition in this question, and most who gained some credit did so by attempting to use a nucleophilic substation mechanism as in part (a). This example scored 2 marks, as the candidate shows all three steps of the SN2 mechanism, which although not correct was worth some credit in the context of this predict question. Try to apply the mechanisms you learn in different contexts. For instance, here the carbonyl group is a pointer to the nucleophilic addition mechanism learnt when studying aldehydes and ketones. GCE Chemistry 9CH

48 Question 9 (a) (i) The K c expression was nearly always correct. When the mark was not awarded it tended to be for use of I and H rather than I 2 and H 2. This candidate has used incorrect formulae for iodine and hydrogen so does not score. Check that the formulae you use in a K c expression match those in the balanced equation. Question 9 (a) (ii) Dealing with a ratio other than 1:1 in equilibria has always provided a degree of challenge and this was evident in part (ii). Most candidates managed to correctly deduce the numerator. However, far fewer could process the information to find the denominator. The most common incorrect responses were a 2 or (a-2y) 2. Only the numerator is correct so this example scores 1 mark. Practice finding concentrations at equilibrium for a variety of reactions, not just those with a 1:1 ratio. 48 GCE Chemistry 9CH0 03

49 Question 9 (b) Most candidates successfully completed the table in part (i), and as a result plotted the graph with care. However, a minority did not extrapolate their line of best fir through (0,0) and so lost 1 mark. It was common to see gradients calculated correctly, within the acceptable range. On occasions, candidates lost marks because they simply chose a point from each axis to determine a gradient. Others failed to show their working on the graph, and as a result it was not always clear to examiners how their value was derived. The determination of K c in part (iv) discriminated effectively, and a significant minority managed to make the gradient the subject of the formula, even if they could not process the data any further. A number ignored the stem of the question and tried to determine K c inserting a pair of values from the table into the expression. GCE Chemistry 9CH

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51 Here, the table is completed correctly and the data plotted with care. However the line of best fit does not extend to (0,0), so a mark was lost. Clear working is shown for the selection of co-ordinates to calculate the gradient in part (iii). Unfortunately in part (iv) the candidate could not manage to make y/a the subject of the formula, so did not find K c. Always use as large a section of your graph as possible when finding a gradient. Show your working clearly on the graph. GCE Chemistry 9CH

52 Question 9 (c) The large number of potential hazards associated with the substances used in this experiment meant most scored the mark. A number missed out because they focussed on the precautions they would take rather than the specific safety risk. The idea of the high pressures was not enough to score a mark. In a chemistry experiment always consider the risks associated with the substances being used. Question 9 (d) Candidates not spotting that a comment was required on both the rate and K c was common. Those who did, often scored both marks. It was also quite common to see candidates try to explain their deductions, which was not required. This example shows a candidate who correctly realises the pressure will increase and considers the effect this has on K c. However there is no discussion of rate, so they only score 1 mark. Read even short questions with care to make sure you are answering all parts. 52 GCE Chemistry 9CH0 03

53 Question 9 (e) (i) Most could explain the effect of a temperature increase on K c, but a number failed to score both marks because they described the change in equilibrium position but did not relate it to the exothermic nature of the reaction. The change in K c is incorrect, so no marks were awarded to this example. Make sure you can apply Le Chatelier s principle to both exo- and endothermic reactions and use the outcome to determine changes to K c. GCE Chemistry 9CH

54 Question 9 (e) (ii) Nearly all attempts here were either a steeper straight line or a less steep straight line. Many candidates, such as this one, thought a steeper line indicated a greater value for the equilibrium constant. Remember that questions in the same section e.g. (i), (ii) etc. often have linked ideas. 54 GCE Chemistry 9CH0 03

55 Paper Summary Based on their performance on this paper, candidates are offered the following advice: Read the stem of each question carefully to ensure you focus your response more precisely on what the question is asking. Take care to revise all the reactions of transition metals highlighted in the specification. Make sure you write full descriptions of observations and equations when you carry out practical work on transition metals. Practise applying organic mechanisms to a wider variety of compounds than just the examples covered in your lessons. When revising, work together with peers to check for clarity in your written explanations, especially when justifying how changes in procedure or conditions might influence a practical outcome. Label each step clearly, when you practice unstructured calculations. This will help embed approaches that you can then try to apply to unfamiliar questions. GCE Chemistry 9CH

56 Grade Boundaries Grade boundaries for this, and all other papers, can be found on the website on this link: 56 GCE Chemistry 9CH0 03

57 GCE Chemistry 9CH

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